I'm not a classicist, just a hacker who loves classical languages. I
personally like Goodwin and Gulick for a grammar. I've taken Homeric and Attic
greek, and studied koine Greek on my own, and I've found G&G up to the task,
whatever the form of Greek used. It is well-organized, with many examples, so
when you're confronted with a strange construction, it's not too difficult to
find the 'strange Greek rule number' which applies to the circumstance.
If you only wish to study koine Greek, perhaps a koine-specific grammar is the
way to go. If you fall in love with Greek, as many do, I recommend a more
encompassing Greek grammar such as G&G.
Mary Ann Davidson